A polymer polyol is used as a material for a polyurethane resin for the purpose of improving properties, for example, compressive hardness and durability, of a polyurethane foam, and is obtained by polymerizing an ethylenically unsaturated monomer in a polyol in the presence of a polymerization initiator. In recent years, a polymer polyol having a small average particle diameter has been demanded for the purpose of further improving compressive hardness and elongation at break. Known methods to decrease the particle diameter include a method of increasing a ratio of acrylonitrile as a part of an ethylenically unsaturated monomer used, a method of using a modified polyol in which a part of hydroxyl groups have been allowed to react with a coupling agent (a silicon-containing compound, trialkoxyalkane, dialkoxyalkane, or the like) (see Patent Document 1), and a method of using a macromer containing unsaturated groups, having a high-molecular-weight part with a number-average molecular weight of 6,000 to 100,000 (see Patent Document 2). Further, another example is available, which is a method of using acrylonitrile at an increased ratio, the acrylonitrile being used as a part of an ethylenically unsaturated monomer used (see Patent Document 3). In this method, a seed dispersion composed of preliminarily formed submicron particles is prepared in polymerization in the first step, and a monomer is added to the seed dispersion in the second step so that a desired polymer concentration is obtained. Still further, a method of obtaining particles by using preliminarily-formed submicron particles as cores is known also (see Patent Document 4).    Patent Document 1: U.S. Pat. No. 4,723,026            (corresponding Japanese publication: JP 61(1986)-246231 A)            Patent Document 2: U.S. Pat. No. 5,494,957            (corresponding Japanese publication: JP 5(1993)-247109 A)            Patent Document 3: JP 6(1994)-172462 A    Patent Document 4: EP 786480            (corresponding Japanese publication: JP 9(1997)-309937 A)        